Lightweight aircraft transparencies have been formed by laminating together a plurality of thin plastic sheets. Commonly, a transparency is formed of a desired number of relatively rigid plastic sheets of requisite curvature interspaced by relatively soft plastic interlayer sheets. Because the optical quality required of such a transparency is high and because the plastic sheets are relatively susceptible to damage, specialized fabrication procedures have developed.
In one such fabrication procedure, the desired number of rigid sheets or "lites", e.g., two to four, are cut from 4 foot.times.4 foot (1.3 meter.times.1.3 meter) blanks of plastic sheeting to a size slightly greater than the desired final size of the transparency to be formed. Each of the lites are then individually formed to a preliminary contour by slumping each individual lite to conform to a shaped plate. This is followed by pressing each individual lite between glass pressing plates at elevated temperature and pressure. An appropriate number of the relatively soft plastic interlayer sheets, such as polyurethane and/or plasticized polyvinyl butyral, are then cut to size and pre-stretched to the preliminary contour of the lites. The lites and interlayer sheets are then laid up in their desired sequence and positioned as an assembly between a pair of glass pressing plates having surfaces covered with a release agent. The entire system is then inserted into an autoclave where lamination and forming to the final shape is accomplished. Thereafter, the laminate is cooled, removed from the autoclave, released from the pressing plates, and trimmed to its desired final configuration.
Although useful for particular simple contours of aircraft transparencies, the preceding procedure involves several limitations which would preferably be avoided. For example, each of the lites and interlayer sheets must be individually cut to a preliminary size and then individually formed to a preliminary contour between glass pressing plates. These operations involve substantial manual labor and time. Further, it may be difficult to succeed in forming each lite to a preliminary contour which is sufficiently similar to each of the other lites to minimize the development of thickness non-uniformities in the interlayer sheets laminated therebetween when the assembly of lites and interlayer sheets is subjected to a laminating temperature that causes the interlayer sheets to flow and compensate somewhat for the dissimilarities in contour of adjacent lites in the assembly. Additionally, the final contour of laminates formed by the preceding procedure is limited to the contour attainable in the pressing plates used to impress the final contour. While this limitation does not cause a problem when forming relatively simple contours, it poses considerable difficulty when relatively complex contours are desired, e.g., compound contours, because optically smooth, compound-contoured glass pressing plates are not readily fabricated. Thus, it can be appreciated that a need exists for a more advantageous method of fabricating a curved laminate, particularly one having a complicated shape.